Electrolytic copper plating solution for filling for forming microwiring of copper for ulsi

ABSTRACT

An object of the present invention is to provide an electrolytic copper plating solution which can suppress, upon electrolytic copper plating on a copper seed layer during fabrication of ULSI copper microwiring (damascene copper wiring) having trends to further miniaturization, dissolution of the copper seed layer and accordingly can suppress occurrence of voids on the inner wall of vias/trenches. 
     The present invention provides an electrolytic copper plating solution for filling for forming microwiring for ULSI, characterized in that it has a pH of 1.8 or higher and 3.0 or lower. The electrolytic copper plating solution preferably comprises a saturated carboxylic acid having 1 or more and 4 or less carbon atoms at 0.01 mol/L or more and 2.0 mol/L or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrolytic copper plating solution for filling for forming microwiring of copper for ULSI.

2. Description of the Related Art

Electrolytic copper plating for filling ULSI microwiring is usually carried out with sulfuric acid-based strongly acidic plating solutions (pH of 1.2 or lower). Seed layers upon plating are sputtered copper films, of which thickness in trenches/vias have become very thin due to miniaturization of wiring. Oxidation of the outermost layer of the seed layer is unavoidable because the outermost layer is atmospherically exposed prior to electrolytic plating. Accordingly, the oxidized parts of the seed layer are easily dissolved when it is immersed in a strongly acidic electrolytic plating solution leading to formation of defects on the thin seed layer. When a copper wiring layer is formed by electrolytic copper plating thereafter, the copper plating is unsatisfactorily absent in spots. Particularly, the inner wall of trenches/vias tends to have problematic voids.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electrolytic copper plating solution which can suppress, upon electrolytic copper plating on a copper seed layer during fabrication of ULSI copper microwiring (damascene copper wiring) having trends to further miniaturization, dissolution of the copper seed layer and accordingly can suppress occurrence of voids on the inner wall of vias/trenches.

The present inventors have attempted to suppress dissolution of copper seed layers upon being immersed in the plating solution by using carboxylic acids and the like instead of sulfuric acid usually used for sulfuric acid-based strongly acidic copper plating solutions and to change pH of the plating solution from conventional strong acid to around weak acid. As a result, they have found that formation of voids on the side wall of trenches which may be generated with the sulfuric acid-based strongly acidic plating solutions can be avoided with the carboxylic acid-based plating solution, thereby accomplished the present invention which solves the above-mentioned problems.

Thus, the present invention provides the followings:

-   -   (1) an electrolytic copper plating solution for filling for         forming microwiring for ULSI, characterized in that it has a pH         of 1.8 or higher and 3.0 or lower;     -   (2) the electrolytic copper plating solution for filling for         forming microwiring for ULSI according to the above (1), wherein         it has a pH of 2.0 or higher and 2.2 or lower;     -   (3) the electrolytic copper plating solution for filling for         forming microwiring for ULSI according to the above (1) or (2),         wherein it comprises a saturated carboxylic acid having 1 or         more and 4 or less carbon atoms at 0.01 mol/L or more and 2.0         mol/L or less;     -   (4) the electrolytic copper plating solution for ULSI         microwiring according to the above (3), wherein the carboxylic         acid is acetic acid;     -   (5) a method for electrolytic copper plating for ULSI         microwiring, characterized in that it uses the electrolytic         copper plating solution for filling for forming microwiring for         ULSI according to any of the above (1) to (4); and     -   (6) a ULSI microwiring substrate characterized in that a ULSI         microwiring is formed by the method for electrolytic copper         plating for ULSI microwiring according to the above (5).

In ULSI copper microwiring (damascene copper wiring) fabrication, formation of a copper wiring layer on a copper seed layer using the electrolytic copper plating solution of the present invention can suppress dissolution of the copper seed layer and accordingly formation of voids on the inner wall of vias/trenches.

BRIEF DESCRIPTION OF THE DRAWINGS Detailed Description of the Preferred Embodiments

The electrolytic copper plating solution for filling for forming microwiring for ULSI of the present invention has a pH of 1.8 or higher and 3.0 or lower. Conventional sulfuric acid-based copper plating solutions are strongly acidic at pH 1.2 or lower. However, the present plating solution can have a pH of 1.8 or higher and 3.0 or lower because carboxylic acid such as acetic acid is used instead of sulfuric acid. By employing pH of 1.8 or higher and 3.0 or lower, dissolution of copper seed layers can be suppressed and accordingly formation of voids on the inner wall of vias/trenches can be suppressed. pH is more preferably 2.0 or higher and 2.2 or lower.

If pH is lower than 1.8, copper seed layers are easily dissolved due to low pH and as a result, voids are also easily formed. If pH is higher than 3.0, copper ions in the plating solution may be converted to oxides or hydroxides to cause precipitations.

The carboxylic acid may be any carboxylic acid that can be dissolved in the plating solution and adjust pH to the above range, and is preferably a saturated carboxylic acid having 1 or more and 4 or less carbon atoms such as formic acid, acetic acid, propionic acid, butyric acid, oxalic acid and the like, with acetic acid being particularly preferable.

The plating solution preferably contains the carboxylic acid at 0.01 to 2.0 mol/L and more preferably 0.2 to 1.0 mol/L. The concentration of the carboxylic acid in the plating solution affects filling properties and pH such that the concentration of carboxylic acid at more than 2.0 mol/L reduces pH of the plating solution to lower than 1.8, causing increased formation of voids. When the concentration of carboxylic acid in the plating solution is less than 0.01 mmol/L, the plating solution has a pH of higher than 3.0, which may cause precipitations as described above.

The electrolytic copper plating solution of the present invention is aqueous and may comprise other components such as copper salts, chloride ions, trace additives and the like, which may be well known and are not particularly limited.

Copper salts may include copper sulfate, copper nitrate, copper chloride and the like, with copper sulfate being preferable. The plating solution preferably contains the copper salt at 0.05 to 1.5 mol/L and more preferably 0.2 to 0.8 mol/L.

The plating solution preferably contains chloride ions at a concentration of 0.3 to 3.0 mmol/L and more preferably 1.0 to 2.0 mmol/L.

Trace additives may include promoters, inhibitors, leveling agents and the like.

Promoters may include bis(3-sulfopropyl)-disulfide, disodium salt, 3-mercaptopropanesulfonic acid and the like, which are preferably contained in the plating solution at 1 to 30 mg/L.

Inhibitors may include polyethylene glycol, polypropylene glycol, copolymers thereof and the like, which are preferably contained in the plating solution at 10 to 500 mg/L.

Leveling agents may include Janus Green B, polyethyleneimine, polyvinylpyrrolidone and the like, which are preferably contained in the plating solution at 0.1 to 50 mg/L.

Plating operations using the electrolytic copper plating solution of the present invention are preferably carried out at a bath temperature of 20 to 30° C. in view of stability of bath and deposition speed of copper. The cathode current density is preferably 0.1 to 5 A/dm2.

A material to be plated by electrolytic copper plating is a microwiring substrate such as semiconductor wafers and is preferably a silicon substrate having ULSI microwiring such as trenches/vias onto which a copper seed layer is provided.

The copper seed layer may be formed by well-known methods such as sputtering and electroless plating.

The electrolytic copper plating solution of the present invention allows to carry out plating without voids even when the copper seed layer in trenches/vias has a thickness of 2 nm or less.

EXAMPLES Example 1

A silicon substrate having ULSI microwiring was subjected to electrolytic copper plating using the following plating solution. The silicon substrate, which is to be plated, has fine trench patterns (line width: 180 nm and depth: 500 nm) and a Cu seed layer is provided on its outermost surface by sputtering. The Cu seed layer had a thinnest thickness of 2 nm in trenches.

Composition of plating solution:

Copper (copper sulfate) 0.63 mol/L; Acetic acid 0.5 mol/L HCl 1.4 mmol/L Bis(3-sulfopropyl)-disulfide, disodium salt 10 mg/L Polypropylene glycol 80 mg/L Polyvinylpyrrolidone 10 mg/L pH 2.1

Plating was carried out at 25° C. and 1 A/dm² for 30 seconds.

Cross-sectional SEM observation is shown in FIG. 1. Void was not formed at anywhere including the side wall part of trenches.

Example 2

A silicon substrate having ULSI microwiring was subjected to electrolytic copper plating using the following plating solution. The silicon substrate to be plated was the same as the one used in Example 1, in which Cu seed layer had a thinnest thickness of 2 nm in trenches.

Composition of plating solution:

Copper (copper sulfate) 0.63 mol/L; Formic acid 1.0 mol/L HCl 1.4 mmol/L Bis(3-sulfopropyl)-disulfide, disodium salt 10 mg/L Polypropylene glycol 80 mg/L Polyvinylpyrrolidone 10 mg/L pH 1.9

Plating was carried out at 25° C. and 1 A/dm² for 30 seconds.

Cross-sectional SEM observation showed that void was not formed at anywhere including the side wall part of trenches.

Example 3

A silicon substrate having ULSI microwiring was subjected to electrolytic copper plating using the following plating solution. The silicon substrate to be plated was the same as the one used in Example 1 except that its Cu seed layer had a thinnest thickness of 1.8 nm in trenches.

Composition of plating solution:

Copper (copper sulfate) 0.63 mol/L; Oxalic acid 0.1 mol/L HCl 1.4 mmol/L Bis(3-sulfopropyl)-disulfide, disodium salt 10 mg/L Polypropylene glycol 80 mg/L Polyvinylpyrrolidone 10 mg/L pH 2.5

Plating was carried out at 25° C. and 1 A/dm² for 30 seconds.

Cross-sectional SEM observation showed that void was not formed at anywhere including the side wall part of trenches.

Comparative Example 1

Electrolytic copper plating was carried out in the same manner as Example 1 except that composition of the plating solution was changed as follows.

Composition of plating solution:

Copper (copper sulfate) 0.63 mol/L Sulfuric acid 0.5 mol/L HCl 1.4 mmol/L Bis(3-sulfopropyl)-disulfide, disodium salt 10 mg/L Polypropylene glycol 80 mg/L Polyvinylpyrrolidone 10 mg/L <pH 1.0

Cross-sectional SEM observation is shown in FIG. 2. Voids (dark shadowy parts in circles) were observed in at least some of the side wall part of trenches. 

1. An electrolytic copper plating aqueous solution for filling for forming damascene microwiring for ULSI in which the copper seed layer in trenches/vias has a thickness of 2 nm or less, characterized in that it has a pH of 1.8 or higher and 3.0 or lower wherein, the plating solution contains copper sulfate at 0.05 to 1.5 mol/L, and the plating solution contains chloride ions at a concentration of 0.3 to 3.0 mmol/L, and the plating solution it comprises a saturated carboxylic acid at 0.01 mol/L or more and 2.0 mol/L or less.
 2. The electrolytic copper plating aqueous solution for filling for forming damascene microwiring for ULSI according to claim 1, wherein it has a pH of 2.0 or higher and 2.2 or lower.
 3. The electrolytic copper plating aqueous solution for filling for forming damascene microwiring for ULSI according to claim 1, wherein saturated carboxylic acid has 1 or more and 4 or less carbon atoms.
 4. The electrolytic copper plating aqueous solution filling for forming damascene microwiring for ULSI microwiring according to claim 3, wherein the carboxylic acid is acetic acid.
 5. A method for electrolytic copper plating for ULSI microwiring, characterized in that it uses the electrolytic copper plating aqueous solution for filling for forming damascene microwiring for ULSI according to claim
 1. 6. A ULSI microwiring substrate characterized in that a ULSI microwiring is formed by the method for electrolytic copper plating aqueous solution for filling for forming damascene microwiring for ULSI according to claim
 5. 